Provide a default to the second argument, as per Rails 3.x

Author: Chris Thompson

lib/polymorphic_integer_type/extensions.rb

@@ -54,7 +54,7 @@ def has_many(name, options = {}, &extension)
         super(name, options, &extension)
       end
 
-      def has_one(name, options)
+      def has_one(name, options = {})
         remove_type_and_establish_mapping(name, options)
         super(name, options)
       end